The invention relates to moldable, thermoplastic multipolymer blends of an acrylic-based copolymer and a polyetheresteramide which exhibits good transparency, good chemical resistance and the enhanced dissipation of electrostatic charge.
Transparent plastic compositions are useful for a number of applications including components for electronic and scientific equipment, point of purchase displays, packaging containers, etc., since they permit visual inspection of the items housed within the plastic. Such transparent plastic compositions are especially useful for the packaging of electronic components such as magnetic head assemblies and clean room applications. In addition, it is often desirable that the packages be reusable after cleaning. However, such transparent plastic materials typically have low conductivity and, as a result, tend to accumulate static electrical charges during their manufacture and use. Such static electrical charges may cause dust or other particles to adhere to the plastic material or even cause the plastic material to adhere to itself or other articles. Further, such static charges may also lead to functional damage in the performance of highly sensitive electronic components.
Accordingly, there is a need for substantially transparent moldable, thermoplastic multipolymer compositions which prevent the buildup of static electrical charges and can dissipate such charges in order to avoid the disadvantages of prior art compositions.
Electrostatic-dissipative multipolymer blends are known in the prior art. For example, U.S. Pat. No. 5,298,558 discloses a blend of polyvinyl chloride, a small amount of an impact modifier polymer such as ABS graft copolymer and an electrostatic dissipative amount of a chain-extended polyether. U.S. Pat. No. 4,775,716 discloses an antistatic thermoplastic composition of an ABS graft copolymer and an electrostatic dissipative composition comprising a copolymer of an epihalohydrin and an oxirane-containing comonomer.
EP 0 596 704 discloses electrostatic-dissipative polymer blends of a styrenic polymer such as ABS or MABS, an epihalohydrin copolymer and a polyalkylenelactone. WO 95/14058 discloses an antistatic thermoplastic composition comprising a MABS copolymer and an inherently antistatic thermoplastic urethane copolymer. Other prior art which mention the use of polyurethanes to confer electrostatic-dissipative properties on thermoplastic polymers include U.S. Pat. Nos. 5,159,053 and 4,179,479.
It is well known that when blending two polymers which are incompatible, the blend will contain large particles or xe2x80x9cislandsxe2x80x9d of one polymer (the minor component) imbedded in the other polymer (the major component) with very little adhesion between the two polymers. This results in undesirable physical properties with the blend often exhibiting the worst properties of both components. It is well known that the use of a compatibilizer, i.e., a xe2x80x9cbridge,xe2x80x9d allows the components to accept each other, thereby resulting in much smaller particles on the minor component, with good adhesion occurring between the two components, thus allowing stress transfer and hence better physical properties.
In the present invention, no compatibilizer is required because there is good inherent compatibility between the methyl methacrylate copolymer and the polyetheresteramide. This is readily seen from the data presented below in Table III in respect to the elongation @ break which is a measure of the toughness of the blend; the data below shows a comparison of the elongation @ break for a blend of a standard grade of a polymer of methyl methacrylate containing a small amount of methyl acrylate comonomer blended with a polyetheresteramide vs. a blend of the methyl methacrylate copolymer and the polyetheresteramide of the present invention.
In accordance with a first aspect of the invention, there is provided a substantially transparent, moldable, thermoplastic multipolymer composition comprising a blend of (a) a methyl methacrylate copolymer and (b) an effective amount of a polyetheresteramide to enhance the electrostatic charge dissipation of the copolymer, said polyetheresteramide having a refractive index within about 0.005 units of the refractive index of the copolymer. In a more preferred embodiment, an impact modifier having a refractive index within about 0.005 units of the refractive index of the copolymer is incorporated in the multipolymer composition.
In accordance with a second aspect of the present invention, there is provided a method for improving the electrostatic charge dissipation of the methyl methacrylate copolymer comprising the step of blending the methyl methacrylate copolymer (and optionally with an impact modifier having a refractive index within about 0.005 units of the refractive index of the copolymer) with an effective amount of a polyetheresteramide having a refractive index within about 0.005 units of the refractive index of the copolymer.
In accordance with a third aspect of the present invention, there is provided a method for preparing a substantially transparent, moldable, thermoplastic multipolymer composition comprising the step of melt blending the methyl methacrylate copolymer with a polyetheresteramide having a refractive index within about 0.005 units of the refractive index of the copolymer at a temperature above the melting temperatures of the copolymer and the polyetheresteramide (and above the melting temperature of the impact modifier having a refractive index within about 0.005 units of the refractive index of the copolymer if it is included).
The substantially transparent, moldable, thermoplastic multipolymer composition of the invention comprises a blend of:
(a) a methyl methacrylate copolymer of a predominant amount of methyl methacrylate and a minor amount of one or more ethylenically unsaturated monomers; and styrene, acrybonitride, methyl acrylate, ethylacrylate
(b) an effective amount of a polyetheresteramide to enhance the electrostatic charge dissipation of the copolymer, said polyetheresteramide having a refractive index within about 0.005 units of the refractive index of the copolymer, such that when the composition is injection molded into a 0.125 inch thick plaque, the plaque exhibits a haze of not greater than about 25% and a light transmission of at least about 60%.
The methyl methacrylate copolymer employed in the compositions of the present invention will contain a predominant amount, e.g., about 50 to about 90 parts by weight, preferably 50 to 80 parts by weight, of methyl methacrylate and a minor amount, e.g., about 10 to about 50 parts by weight, preferably 20 to 40 parts by weight, of one or more ethylenically unsaturated monomers such as styrene, acrylonitrile, methyl acrylate, ethyl acrylate and mixtures thereof. Preferably, the ethylenically unsaturated monomer comprises a mixture of styrene and acrylonitrile or styrene and ethyl acrylate wherein the styrene is present in the copolymer in an amount of about 10 to about 40, preferably 15 to 30, parts by weight and the acrylonitrile is present in the copolymer in an amount of about 5 to about 30, preferably 5 to 20, parts by weight, based on the weight of the copolymer or the ethyl acrylate is present in the copolymer in an amount of about 3 to about 10, preferably 5 to 10 parts by weight, based on the weight of the copolymer. Such methyl methacrylate copolymers are well known in the prior art, e.g., U.S. Pat. Nos. 3,261,887; 3,354,238; 4,085,166; 4,228,256; 4,242,469; 5,061,747; and 5,290,860.
Preferably, the methyl methacrylate copolymer will have a weight average molecular weight of at least about 50,000, e.g., about 100,000 to about 300,000 and a glass transition temperature of at least about 50xc2x0 C. Typically, the methyl methacrylate copolymer will have a refractive index of about 1.50 to about 1.53, preferably 1.51 to 1.52, (as measured in accordance with ASTM D-542).
The multipolymer compositions of the invention will also contain an effective amount of a polyetheresteramide to enhance the electrostatic charge dissipation of the copolymer. The polyetheresteramide should have a refractive index within about 0.005 units, preferably within 0.003 units, of the refractive index of the copolymer (as measured in accordance with ASTM D-542). Typically the polyetheresteramide will be present in the amount of about 5 to about 35, preferably 10 to 30, wt. %, based on the weight of the composition. The resultant composition when injection molded into a plaque having a thickness of 0.125 inch will be such that the plaque exhibits a haze of not greater than about 25%, preferably not greater than 15% (as measured in accordance with ASTM D-1003) and a light transmission of at least about 60%, preferably at least 80% (as measured in accordance with ASTM D-1003).
Polyetheresteramides are well known in the prior art, e.g, see U.S. Pat. Nos. 4,376,856; 4,438,240; 4,536,530; 4,689,393; 5,096,995; 5,331,061; 5,604,284; 5,652,326; and 5,886,098. Polyetheresteramides useful in preparing the composition of the present invention are commercially available from Sanyo Chemical Industries under the brand name xe2x80x9cPelestatxe2x80x9d in a variety of grades having refractive indices ranging from about 1.49 to about 1.53.
The composition of the present invention may also contain a polyethylene glycol in order to improve the chemical resistance of the composition and permit the use of lower levels of the polyetheresteramide in the composition. The polyethylene glycol, if used, will have a weight average molecular weight of about 2,000 to about 10,000, preferably 3,000 to 6,000 and in an amount of about 1 to about 10 wt. %, preferably 2 to 6 wt. %, based on the weight of the copolymer plus the polyetheresteramide plus the polyethylene glycol.
Preferably, the composition of the present invention includes an impact modifier having a refractive index within about 0.005 units, preferably within 0.003 units, of the refractive index of the methyl methacrylate copolymer (as measured in accordance with ASTM D-542). Typically, the impact modifier will be present in an amount of about 2 to about 30, preferably 5 to 20 wt. %, based on the weight of the copolymer plus the polyetheresteramide plus the impact modifier.
Preferable impact modifiers for incorporation in the multipolymer compositions of the present invention include copolymers of a conjugated diene rubber grafted with one or more ethylenically unsaturated monomers as well as acrylic copolymers having a core/shell structure.
In the case where the impact modifier comprises a copolymer of the conjugated diene rubber, the rubber is preferably polybutadiene which is present in an amount of about 50 to about 90, preferably 70 to 80, parts by weight, based on the weight of the impact modifier, and the ethylenically unsaturated monomer(s) grafted onto the polybutadiene rubber is typically present in an amount of about 10 to about 50, preferably 15 to 40, parts by weight, based on the weight of the impact modifier. Typically, the ethylenically unsaturated monomer to be grafted onto the conjugated diene rubber will be a C1-C4 alkyl acrylate such as methyl acrylate, ethyl acrylate, propyl acrylate or butyl acrylate; a C1-C4 alkyl methacrylate such as methyl methacrylate, ethyl methacrylate, propyl methacrylate or butyl methacrylate; a styrene such as styrene or xcex1-methyl styrene; a vinyl ether; a vinyl halide such as vinyl chloride; a nitrile such as acrylonitrile or methacrylonitrile; an olefin or mixtures thereof. Preferably the ethylenically unsaturated monomer(s) to be grafted onto the conjugated diene rubber comprises a monomer mixture of methyl methacrylate and styrene, with the methyl methacrylate: styrene ratio being in the range of about 2:1 to about 5:1, preferably 2.5:1 to 4.5:1.
In the case where the impact modifier comprises an acrylic copolymer having a core/shell structure, it is preferred that the core/shell structure comprises a core of a cross-linked poly(alkylmethacrylate) or a cross-linked diene rubber and a shell of a copolymer of an alkyl acrylate (e.g., methyl acrylate) and styrene. It is further preferred that the poly(alkyl-methacrylate) comprises poly(methyl methacrylate), the diene rubber comprises polybutadiene rubber and the alkyl acrylate comprises butyl acrylate. It is especially preferred that there be an additional outer shell of poly(methyl methacrylate) in addition to the shell of the alkyl acrylate/styrene copolymer.
The method of enhancing the electrostatic charge dissipation of a substantially transparent, moldable, thermoplastic methyl methacrylate copolymer comprises the step of blending the copolymer (possibly also with a polyethylene glycol and preferably also with an impact modifier) with an effective amount of a polyetheresteramide to enhance the electrostatic charge dissipation of the copolymer. The copolymer, polyethylene glycol and impact modifier (if used) and the polyetheresteramide and the amounts thereof are as described above. Preferably, the copolymer, polyethylene glycol and/or impact modifier (if used) and the polyetheresteramide are blended by melt blending the components at a temperature above the melting temperatures of the components.